diff --git a/source/conf.py b/source/conf.py

index e1430c13..6028aa21 100644

--- a/source/conf.py

+++ b/source/conf.py

@@ -181,4 +181,4 @@ texinfo_documents = [

 ]

 def setup(app):

-    app.add_stylesheet('css/theme_extra.css')  # may also be an URL

+    app.add_css_file('css/theme_extra.css')  # may also be an URL